Yeah there aren't a lot of them, unfortunately. And those that do feature a bi MC are usually quite heavy on furry stuff for some reason. But if you don't mind some furries there's
Broken Dreams Correctional Center and
Lilith's Throne (both let you choose your gender and whether you're straight, gay or bi) as well as all the
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titles. There's also
Debasing Grounds, a text-based thing that is in early development. No furries, but not a lot of content either - and quite buggy.